Trichromatic manipulation of dispersion switching, enhancement of Kerr nonlinearity and population inversion are analyzed in a three-level Λ atomic system. It is shown that, due to the interference among multiple two-photon channels, the Kerr nonlinearity is enhanced in multiple transparent windows and the frequency band corresponding to population inversion becomes wider. On the other hand, the sum of the relative phases of the sideband components of the trichromatic field plays a crucial role in the response of the medium. As the sum phase is adjusted from 0 to π, dispersion is switched from normal to anomalous in multiple frequency regimes, which corresponds to the switching from subluminal to superluminal light propagation. With the sum phase changing from 0 to π, the Kerr nonlinearity is enhanced in other different frequency regimes, and the frequency band of population inversion is further widened. The influence mechanism of the sum phase is also analyzed.